Sustainable low temperature desalination: A case for renewable energyVeera Gnaneswar Gude, Nagamany Nirmalakhandan, and Shuguang Deng

Citation: Journal of Renewable and Sustainable Energy 3, 043108 (2011); doi: 10.1063/1.3608910 View online: http://dx.doi.org/10.1063/1.3608910 View Table of Contents: http://scitation.aip.org/content/aip/journal/jrse/3/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A sustainable renewable energy mix option for the secluded society J. Renewable Sustainable Energy 6, 023124 (2014); 10.1063/1.4873129 Exceptional ion rejection ability of directional solvent for non-membrane desalination Appl. Phys. Lett. 104, 024102 (2014); 10.1063/1.4861835 Potential role of renewable energy in water desalination in Australia J. Renewable Sustainable Energy 4, 013108 (2012); 10.1063/1.3682060 Industrial effluent treatment: Theoretical and experimental analysis J. Renewable Sustainable Energy 3, 013107 (2011); 10.1063/1.3558862 SolarPowered Desalination: A Modelling and Experimental Study AIP Conf. Proc. 941, 249 (2007); 10.1063/1.2806091

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Sustainable low temperature desalination: A case forrenewable energy

Veera Gnaneswar Gude,1,a) Nagamany Nirmalakhandan,2 andShuguang Deng1

1Chemical Engineering Department, New Mexico State University,Las Cruces, New Mexico 88001, USA2Civil Engineering Department, New Mexico State University, Las Cruces,New Mexico 88001, USA

(Received 9 August 2010; accepted 10 June 2011; published online 27 July 2011)

In this paper, different configurations for running a low temperature desalination

process at a production capacity of 100 liters=day are presented. Renewable energy

sources such as solar and geothermal energy sources are evaluated as renewable,

reliable, and suitable energy sources for driving the low temperature desalination

process round the clock. A case study is presented to evaluate the feasibility of

sustainable recovery of potable water from the effluent streams of wastewater

treatment plant. Results obtained from theoretical and experimental studies

demonstrate that the low temperature desalination unit has the potential for large

scale applications using renewable energy sources to produce freshwater in a

sustainable manner. The following renewable energy=waste heat recovery

configurations may produce around 100 liters=day of desalinated water: (1) solar

collector area of 18 m2 with a thermal energy storage (TES) volume of 3 m3; (2)

photovoltaic thermal collector area of 30 m2 to provide 14–18 kW electricity and

120 liters=day freshwater with an optimum mass flow rate of the circulating fluid

around 40–50 kg=h m2; (3) A geothermal source at 60 �C with a flow rate of 320

kg=h; and (4) waste heat rejected from the condenser of an absorption refrigeration

system rated at 3.25 kW (0.95 tons refrigeration), supported by 25 m2 solar

collector area and 10 m3 TES volume. Additionally, the secondary effluent of local

wastewater treatment plant was processed to recover potable quality water.

Experimental results showed that >95% of all the water contaminants such as

biological oxygen demand (BOD), total dissolved solids (TDS), total suspended

solids (TSS), ammonia, chlorides, nitrates, and coliform bacteria can be removed

to provide clean water for many beneficial uses. VC 2011 American Institute ofPhysics. [doi:10.1063/1.3608910]

I. INTRODUCTION

In many parts of the world, desalination has become an imperative and inevitable solution

to overcome the shortage of potable water. Current desalination technologies are based on ther-

mal evaporation or membrane separation principles. Thermal desalination technologies require

large quantities of energy and fossil fuels have traditionally been used to provide the energy

requirements for desalination of seawater or brackish waters. The idea of utilizing the fossil

fuels to produce freshwater through desalination processes is not a sustainable approach any

more due to the rapid decline in these resources and resultant high fuel costs and negative envi-

ronmental impacts. In an effort to conserve the depleting natural fossil fuel resources, desalina-

tion industry has been adopting several energy-saving measures in recent years. Examples

a)Author to whom correspondence should be addressed. Present address: Civil Engineering Department, Oregon Institute

of Technology, 3201 Campus Drive, Klamath Falls, Oregon 97601, USA. Electronic mail: gudevg@gmail.com. Tel.:

1(530) 751 6061. FAX: 1(575) 646 7706.

1941-7012/2011/3(4)/043108/25/$30.00 VC 2011 American Institute of Physics3, 043108-1

JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 3, 043108 (2011)

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include recovery and recycling of energy as in the case of staging, low temperature desalina-

tion, and utilization of waste heat or renewable energy.

Various renewable energy resources are available that suit the energy needs of different

desalination processes. Currently, the resources which are well explored and exploited for

desalination applications include solar energy (harvested by solar collectors or photovoltaic

modules), wind energy, geothermal energy, and wave energy.1 Sustainable use of these resour-

ces depends on the conversion technology employed and the end-user process configuration.

Sustainability, in this context, can be interpreted as how the available energy resource is being

utilized. Conserving, recycling and increasing the efficiency of the conversion technologies are

some approaches which result in a sustainable use of an energy resource. For instance, daily so-

lar energy available on the surface of the earth is roughly 15 000 times greater than the daily

energy consumption of the world population,2 which means that only much less than 1% of the

daily solar energy captured in energy devices could solve the energy problems of the world.

However, the maximum energy conversion rate of the current photovoltaic modules in the mar-

ket has not exceeded 15% to date. This indicates that the energy harvested through these

resources, though freely available, still have a very high value and need to be utilized effi-

ciently or utilized in a “sustainable manner”.

One way to utilize the renewable energy sources more efficiently is by coupling with an

energy-efficient desalination process. Thermal desalination processes operating at higher tem-

peratures such as multi-stage flash distillation (MSF), multi-effect distillation (MED) technology

require high quality heat sources at higher temperatures and result in higher fugitive losses and

consumption of prime non-renewable energy sources. On the other hand, low temperature

desalination processes have lower specific energy requirements and a higher thermodynamic ef-

ficiency. Apart from the above, other advantages include lower corrosion rates, low-cost materi-

als of construction with a longer plant life, lower scaling, lower heat losses, and shorter start-up

periods. The motive energy for driving the low temperature processes can be provided by low

grade heat sources (renewable energy) or process waste heat rejections, so that better economies

of the overall processes can be achieved.3–5

In this research, a new low temperature desalination process has been developed which can

utilize low grade heat sources such as waste heat releases, solar, photovoltaic=thermal

(PV=Thermal), and geothermal energy sources. Since the process operates at lower tempera-

tures, energy losses and, hence, the energy requirements for desalination are reduced. As this

process utilizes renewable energy and waste heat releases, it does not directly contribute to any

greenhouse gas emissions and can be considered a sustainable process. Results obtained from

theoretical modelling studies and experimental studies are presented in this paper to demon-

strate the viability of the proposed desalination process. Different configurations in which the

proposed process can be driven using different energy sources at a desalination production

capacity of 100 liters=day and the energy requirements are discussed. This paper focuses on

theoretical development of the low temperature desalination system using different renewable

energy sources with a limited analysis of experimental results.

A. Description of the desalination system

The premise of the proposed system can be explained by considering two barometric col-

umns at ambient temperature, one filled with freshwater and the other with saline water as

shown in Fig. 1. The barometric columns contain the head equivalent to local atmospheric pres-

sure and when closed, a vacuum will be created in the headspace by the amount of the fluid

volume displaced by gravity. Due to the natural vacuum generated by this process, the head

space of these two columns will be occupied by the vapors of the respective fluids at their re-

spective vapor pressures. If the two head spaces are connected to one another, water vapor will

distill spontaneously from the freshwater column into the saline water column, because the

vapor pressure of freshwater is slightly higher than that of saline water at ambient temperature.

However, if the temperature of the saline water column is maintained slightly higher than that

of the fresh water column to raise the vapor pressure of the feed water side above that of the

043108-2 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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fresh water side, water vapor from the saline water column will distill into the fresh water col-

umn. A temperature differential of about 10–15 �C is adequate to overcome the vapor pressure

differential to drive this desalination process. Such low temperature differentials can be

achieved using low grade heat sources such as solar energy, process waste heat, thermal energy

storage (TES) systems, etc.

A schematic arrangement of the low temperature desalination system based on the above

principles is shown in Fig. 2. Components of this unit include an evaporation chamber (EC), a

natural draft condenser, two heat exchangers, and three barometric columns. These three col-

umns serve as the saline water column, the brine withdrawal column, and the desalinated water

column, each with its own holding tank, SWT (seawater tank), BT (brine tank), and DWT

(desalinated water tank), respectively. The brine tank holds the concentrate removed from the

evaporation chamber to maintain the salt concentration in the evaporation chamber. The

FIG. 1. Physical principle of the low temperature desalination system.

FIG. 2. Low temperature desalination system powered by renewable and waste heat sources.

043108-3 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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evaporation chamber can be designed to use direct solar energy (with glass top exposed to solar

radiation) and waste heat sources (as shown in Fig. 2).

The EC is installed atop the three columns at a height of about 10 m above ground level

to create vacuum naturally in the headspaces of the feed, withdrawal, and desalinated water

columns. This configuration drives the desalination process without any mechanical pumping.6

The saline water enters the evaporation unit through a tube-in-tube heat exchanger.1,2 The

temperature of the head space of the saline water column is maintained slightly higher than

that of the desalinated water column. Since the head spaces are at near-vacuum level pres-

sures, a temperature differential of 10 �C is adequate to evaporate water from the saline water

side and condense in the distilled water side.3–5 In this manner, saline water can be desali-

nated at about 40–50 �C, which is in contrast to the 60–100 �C range in traditional solar stills

(SSs) and other distillation processes. This configuration enables the brine to be withdrawn

continuously from the EC through heat exchanger 1 (HE1), preheating the saline water feed

entering the EC.6,7 Further, by maintaining constant levels of inflow and outflow rates in

SWT, BT, and DWT, the system can function without any energy input for fluid transfer in

the desalination system. The heat input to EC is provided by TES tank through a heat

exchanger 2 (HE2) which in turn is fed by a low grade waste heat or renewable energy

source. Different heat sources evaluated in this study are solar collectors, photovoltaic thermal

collectors, geothermal energy sources, and process waste heat. Experimental results obtained

for a configuration using glass top evaporation chamber to utilize direct solar energy were

compared with theoretical results obtained in Sec. III. A closed top evaporation chamber was

also tested for recovering potable quality water from the secondary effluent of the local waste-

water treatment plant.

B. Theoretical analysis of the desalination system

Mass and energy balances around the EC yield the following coupled differential equations,

where the subscripts refer to the state points shown in Fig. 2. The variables are defined in the

Appendix.

Mass balance on volume of water in EC,

d

dtðqVÞ ¼ m2 � m6 � m3: (1)

Mass balance on solute in EC,

d

dtðqVCÞEC ¼ m2C2 � m6C6: (2)

Energy balance for volume of water in EC,

d

dtðqVcpTÞEC ¼ QEC þ ðmcpTÞ2 � ðmcpTÞ6 � m3hLðTÞ � Ql; (3)

where QEC is the rate of energy input (load on the TES) to the EC and Ql is the rate of energy

loss from the EC. The energy input, QEC, to the evaporation chamber can be supplied by the

solar collectors, photovoltaic thermal collectors, geothermal water sources, and process waste

heat sources to the TES as discussed in Sec. I A and is written as

QEC ¼ mscsðTs � TECÞ; (4)

where, ms, cs, and Ts, are the mass flow rate, specific heat, and temperature, respectively, of the

water from the TES and TEC is the temperature of the saline water in the evaporation chamber.

Theoretical expressions for the different heat sources are discussed next.

Desalination efficiency is defined as

043108-4 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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g ¼MhLðTÞ

RðQECDtÞ ; (5)

where

hLðTÞ ¼ 3; 146� 2:36ðT þ 273Þ: (6)

Evaporation rate as a function of pressure and temperature,8

m3 ¼ AECk fðCECÞpðTECÞ

ðTEC þ 273Þ0:5�

pðT5Þ

ðT5 þ 273Þ0:5

" #; (7)

where

pðTÞ ¼ ½expð63:02� 7139:6=ðT þ 273Þ � 6:2558 lnðT þ 273Þ� � 102Pa: (8)

The above coupled equations are solved using Extend (Imagine That Inc.) and Engineering

Equation Solver (EES) simulation software. Details of heat transfer relations for evaporation

chamber and condensation surface and heat losses by convection and radiation are presented

elsewhere.7,8 Model parameters for the low temperature desalination process are presented in

Table I.

II. THEORETICAL STUDIES

In this section, theoretical analyses for different energy sources and the results from the

modeling studies are presented. The expressions for different energy sources can be substituted

in the overall energy balance (Eq. (3)) to generate the simulations. Schematics for different con-

figurations are shown in Fig. 3.

A. Solar collectors

Flat plate solar collectors supplying low grade heat in the range of 50–70 �C can be used

to drive the proposed desalination system during sunlight hours (Fig. 3(a)). The sensible heat

stored in the TES will provide the heat source to the evaporation chamber during non sunlight

hours.

Energy balance across the solar panel can be written as

dðmcTÞsc

dt¼ FRAC½ðsaÞIs � ULðTSC � TaÞ � QS; (9)

where Qs is the solar energy harvested by the solar collectors and stored in the TES tank and is

given as

Qs ¼ mRcRðTSC � TsÞ; (10)

TABLE I. Model parameters for the low temperature desalination system.

Parameter Value Parameter Value

EC area m2 1–5 Solar insolation W=m2 200-1000

Condenser area m2 1–5 Seawater concentration % 3.5

Water depth in the EC m 0.05-0.1 Seawater density kg=m3 1020

Height of EC m 0.5 Seawater, TES reference Temperature �C 25

TES volume m3 1 Ambient temperature �C �3 to 35

043108-5 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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where mR and cR are the mass flow rate of the collector fluid, Tsc is the temperature of the

water exiting the solar collector, and Ts is the temperature of TES tank. The energy balance on

the TES can be written as8

d

dtðMcTÞs ¼ Qs � QEC � Qls; (11)

FIG. 3. Energy balance on different heat sources: (a) solar collectors, (b) photovoltaic thermal collectors, (c) geothermal

source, and (d) process waste heat.

043108-6 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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where Ms (q�) is the total mass of water in the TES, cs, and Ts are as defined earlier, and Qs is

the thermal energy supplied by the solar collectors. Actual energy supplied from the TES tank

to the evaporation chamber, QEC can be calculated using Eq. (4). Qls are the energy losses from

the TES.

1. Performance of the low temperature desalination system

Temperature profiles of the desalination system driven by the solar collectors are shown in

Fig. 4(a) for both evaporation and solar collector areas of 1 m2. The TES temperatures reach a

maximum value of 57 �C during sunlight hours and evaporation temperatures reach a maximum

value of 45 �C. The maximum ambient temperature is 34 �C. The energy supplied from the

TES and the energy utilized for evaporation are shown in Fig. 4(b). Hourly freshwater produc-

tion rates and cumulative product are shown in Fig. 4(c). A daily product of 6.5-7 liters can be

produced for 1 m2 evaporator and 1 m2 solar collector areas. These results are compared to

those reported previously.6 The evaporation efficiency of the process ranged between 60% and

90%, most of the time in the range 75%-90% as shown in Fig. 4(d). The TES volume used in

this simulation was 0.1 m3. Hourly and daily freshwater production rates are shown for a solar

collector area of 18 m2 and a TES volume of 3 m3 in Fig. 5. As it can be seen from Fig. 5, the

freshwater production rates for this case start to stabilize after 72 h of operation. The freshwater

production rate was lower during initial hours as some of the energy supplied from the solar

collectors is utilized to increase the sensible heat of the total volume of the water in TES. The

stabilized freshwater production rate for this configuration is 104 liters=day.

2. Use of Thermal Energy Storage System

In this configuration, the need for thermal energy storage tank was evaluated through simu-

lations. Fig. 6 shows the TES performance for two different volumes (1 m3 and 6 m3) over 168

h (7 days). The TES temperatures fluctuate in relation with the daily solar insolation and ambi-

ent temperatures in both cases. TES temperatures for a TES volume of 1 m3 respond quickly to

the changes in the solar insolation and ambient temperatures with very little sensible heat avail-

able for desalination during nonsunlight hours. Temperatures during sunlight hours reach as

high as 65–68 �C and fall down to as low as 30–35 �C during nonsunlight hours. TES

FIG. 4. Low temperature desalination system driven by solar energy; (a) temperature profiles, (b) energy utilization, (c)

daily production rates, and (d) desalination efficiency.

043108-7 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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temperatures for a TES volume of 6 m3 respond slowly to the daily solar insolation, and ambi-

ent temperatures increasing sensible heat of the bulk of the water in the tank. For this tank vol-

ume, the TES water temperature during nonsunlight hours was around 45–48 �C, which is still

a good heat source for evaporation during nonsunlight hours enabling 24 h operation. The max-

imum and minimum TES temperatures are shown in Fig. 7 for different TES volumes in the

range 1–6 m3. From Fig. 7, it can be observed that as the TES volume increases the heat source

available for nonsunlight hour operation increases. Relation between the solar collector areas in

FIG. 5. Low temperature desalination system driven by solar collectors round the clock (TES volume: 3 m3, solar collector

area: 18 m2).

FIG. 6. TES performance over 7 days (168 h).

043108-8 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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connection with the TES volumes is presented in Fig. 8. For higher TES volumes, it is obvious

that the solar collector area requirement is high for the freshwater production rate of around

100 liters=day. It is because sensible heat losses to the ambient from the TES tank are to be

provided by the same collector area. Therefore, the collector area required for 1 m3 TES vol-

ume is 15 m2, which is increased by 25% and 40% for TES volumes of 3 m3 and 6 m3, respec-

tively. Hourly and daily freshwater production rates for a fixed solar collector area of 15 m2

over 21 days of operation are shown in Fig. 9. At the end of 7 days of operation, the average

daily freshwater production for TES volume of 1 m3 remained at 100 litres=day and for TES

volume of 6 m3 at 68 liters=day. It should be noted that the average freshwater production rates

continue to increase for the TES volume of 6 m3 and reach 86 liters=day at the end of 21 days

of operation. From these simulations, the maximum TES temperatures for TES volume of 1 m3

remained at 68 �C, whereas for TES volume of 6 m3, the temperatures increased from 53.8 �Cto 54.5 �C at the end of 21 days of operation. This indicates that the energy stored in the TES

is available for longer periods of time enabling continuous and stable freshwater production

FIG. 7. Effect of TES volume on TES and EC temperature profiles.

FIG. 8. TES volume effect on solar collector area and freshwater production rate.

043108-9 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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rates. Therefore, to determine the optimum solar collector areas, long term performance of the

TES needs to be considered. The configuration with small TES volumes will suffer from the

changes in daily solar insolation and ambient temperatures. High TES volume may initially

result in lower freshwater production rates but with continued operation the productivity will be

increased. For small TES volumes, the freshwater production rates cease during nonsunlight

hours leaving the unit idle for 33%–50% of the day. If the high storage volume of TES is a

constraint, freshwater has to be stored in water tank for rest of the day, cloudy, and rainy day

needs as well. Continuous operation allows for downsizing the desalination unit and reduces

the equipment cost. Batch operation requires a large evaporation area and equipment with

higher costs. Continuous process mode is easily adaptable to other low grade waste heat sources

and can be scaled to large applications to provide freshwater for small rural communities.

3. Cloudy Day Effect on Thermal Energy Storage System

The effect of cloudy days on different TES volumes was investigated. One cloudy day per

week was considered over 3 weeks (21 days). As can be seen from Fig. 10, the average daily

production over 21 days of operation decreases from 100 liters=day to 92 liters=day (by 8 liter-

s=day) for TES volume of 1 m3 with a solar collector area of 15 m2. For the same collector

area, the average daily production over 21 days decreased from 93 to 87 (by 6 liters=day) and

86 to 81 liters=day (by 5 liters=day), respectively, for TES of 3 m3 and 6 m3 volumes. Consid-

ering the performance on the cloudy days alone, the reduction in the daily product amount was

100 to 63, 93 to 67, and 86 to 75 liters=day, which are 37%, 28%, and 13% reductions as

shown in Fig. 10. The hourly freshwater production rates decreased from 7.5 to 4.6, 5.1 to 3.5,

and 4.5 to 3.5 liters=h for TES volumes of 1, 3, and 6 m3, respectively, the smallest variation

being observed for 6 m3. Similar comparison for different TES volumes with a solar collector

area of 18 m2 was done over 21 days of operation. Based on this analysis, the daily production

increased for TES volume of 6 m3 from 75 liters=day (15 m2 solar collector area) to 88 liter-

s=day (18 m2 solar collector area). The hourly production rates also stabilized with this configu-

ration. As shown in Fig. 11, the solar collector area requirements based on 21 days operation

are 6% and 13% only compared to 7 days operational performance analysis, 25% and 40% for

TES volumes of 3 and 6 m3, respectively. Therefore, based on the above analysis, it is clear

that the TES performance has to be evaluated on a long term performance basis and the

FIG. 9. TES volume effect on daily production rates and cumulative product over 21 days.

043108-10 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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advantage of the TES is recognized when long term operations are considered. The process

conditions are more stable with variations in energy supply and demand trends. The need for

the TES, however, depends on the type, scale, and economics of a particular application.

B. PV=Thermal collectors

Photovoltaic thermal collectors have the ability to produce both electrical and thermal

energy from solar energy. The overall efficiency of the PV=Thermal collectors is higher than

the sum of the efficiencies of separate photovoltaic modules and solar thermal collectors.9 Ther-

mal energy produced from PV=Thermal collectors is suitable for low temperature desalination

by the proposed process while electricity produced can be used for domestic uses (Fig. 3(b)).

Theoretical analysis for the desalination system utilizing photovoltaic thermal energy is as

follows.

The energy balance on the PV=Thermal collector and the absorber plate can be written as

follows10:

FIG. 11. TES volume effect on solar collector area and freshwater production rate based on 21 days performance.

FIG. 10. TES volume effect on daily production rates and cumulative product over 21 days with cloudy day effects.

043108-11 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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MpCpdTPVT

dt¼ Qsp � Qlp � Pe � Qu; (12)

where Mp and Cp are the mass and specific heat capacity of the absorber plate, respectively,

Qsp is the incident solar energy, Qlp are the losses from the PV=Thermal collector, Pe is the

electrical energy derived from the module, and Qu is the useful energy (thermal) extracted by

the collector fluid.

Solar energy absorbed by the PV=Thermal panel material is given as

Qsp ¼ Issgap: (13)

Heat losses through radiation and convection from the PV=Thermal absorber plate to the glass

cover can be written as10

Qlp ¼ eaegrfT4p � T4

gg þ hpgðTp � TgÞ: (14)

Electrical energy generated by PV=Thermal collector system is given as11

Pe ¼ IssgFcgstdf1� 0:005ðTp � 298:15Þg: (15)

Useful thermal energy derived from PV=Thermal collectors can be expressed as

Qu ¼ mCpf ðTf � TiÞ; (16)

where Tf is the collector fluid exit temperature and Ti is the collector fluid inlet temperature. m

and Cpf are the mass flow rate and specific heat capacity, respectively, of the collector fluid

which is water in this case.

The circulating fluid exit temperature, Tf, can be calculated as follows:10

Tf ¼ ðTPVT � TiÞ 1� exp �4 � ðx=dÞNu

Re: Pr

� �� �þ Ti; (17)

where TPVT is the absorber plate temperature, x and d are the length and diameter of the circu-

lating fluid tube, respectively.

The heat input to the EC is the useful heat extracted from the PV=Thermal collectors and

stored in thermal energy storage tank and is given by

dðvqcTÞsdt

¼ Qu � QEC � Qlo; (18)

where v is the volume of the storage tank, Cps is the specific heat of the water in the storage

tank, and Ts is the storage tank temperature. QEC is the heat supplied to the EC and Qlo is the

energy losses from the storage tank. Actual amount of heat supplied to the EC from TES can

be obtained by using Eq. (4).

Thermal and electrical efficiencies of the PV=Thermal collector at a given time are as

follows:12

Thermal efficiency of the PV=Thermal collector ¼gPVT;th ¼mCpf ðTf � TiÞ

IAc; (19)

Total PV=Thermal collector efficiency ¼gPVT ¼mCpf ðTf � TiÞ þ Pe

IAc; (20)

where Ac is the collector area.

043108-12 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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1. Integrated PV=Thermal-desalination system

Numerical simulations have been performed for a site in southern New Mexico. Parameters

used in the numerical simulations are presented in Table II for the PV=Thermal collector sys-

tem. The solar insolation and ambient temperatures on a summer day in June varied between

225–1000 W=m2 and 16–33 �C, respectively. The temperature profiles for the photovoltaic ther-

mal system are shown in Fig. 12. The absorber plate temperatures reached up to 68.7 �C, while

the collector fluid temperature reached a maximum value of 64.5 �C in the middle of the day.

The glass cover was in contact with the ambient air and reached a maximum temperature of

43.1 �C, while the maximum ambient temperature was 33 �C. In this simulation, the mass flow

rate of the circulating fluid was 40 kg=h m2. The volume of the TES was 1 m3 with a volume

to collector area ratio of around 40 litres=m2.

Thermal and electrical energy production rates and their efficiencies during sunlight hours

are shown in Fig. 13. Thermal energy is the useful energy extracted by the circulating fluid in

the PV=Thermal collector. Total PV=Thermal system efficiency of 59.4% and thermal effi-

ciency of nearly 50.5% are predicted. These results are comparable with the previous stud-

ies.13,14 The primary energy saving efficiency of PV=Thermal system is predicted as 72.7%,

which considers the total energy that will otherwise be required to generate the electricity by a

conventional power plant. A photovoltaic collector (Sharp NT-S5E1U, cell efficiency 17.5%,

and module efficiency 14.2%) was used in these simulations. It is well known that the photo-

voltaic cell efficiency would decrease with increasing absorber plate temperature. The electrical

efficiency of the PV=Thermal system varied between 12.5% and 8.5% during the sunlight

hours.

2. Low Temperature Desalination System Driven by PV=Thermal System

Useful energy extracted from the PV=Thermal system was used as the heat source to drive

the desalination system. The mass flow rate of the circulating fluid between the TES and the

evaporation chamber was fixed at 60 kg=h m2 in these simulations. The resulting evaporation

temperatures in the desalination system and freshwater and ambient temperatures are shown in

Fig. 14. The maximum saline water temperature in the evaporation chamber is predicted as

51.2 �C at the maximum TES and ambient temperatures 57.2 �C and 33 �C, respectively. The

saline water temperature decreased with the collector fluid temperature and eventually reached

ambient temperatures during non-sunlight hours. The useful energy supplied to the evaporation

chamber is utilized for evaporation, with energy losses ranging from 10% to 20% of the total

useful energy supplied, resulting in 80%–90% evaporation efficiencies. The hourly freshwater

production rate is shown in Fig. 15. As expected, the evaporation rate increased with increase

in the heat source temperature and a maximum evaporation rate of 15 liters=h is obtained. The

cumulative amount of freshwater produced from the desalination system under the specific con-

ditions was 120 liters. The optimum PV=Thermal collector area for this application was found

to be 30 m2 which can also provide 14–18 kW h of electricity needs for a household.15 The

mechanical energy required to circulate the collector fluid is calculated as 4 kJ=kg of freshwater

produced.

TABLE II. Model parameters for photovoltaic thermal collector system.

Parameter Value Parameter Value

Cell material Mono-Si Material of absorber aluminum

PV=T module area (m2) variable Absorption factor, PV cell 0.9

Total PV=T module area (m2) 20–30 Radiation factors, cover glass-eg, and absorber ea 0.05, 0.5

Cell efficiency (%)a) 17.5 Absorption factor, cover glass-eg, and absorber ea 0.05, 0.16

Coefficient of temperature inversion (%=�C) 0.5 Collector fluid (kg=h=m2) Water, 1–80

a)At Ta¼ 25 �C, Is¼ 1000 W=m2.

043108-13 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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FIG. 12. Temperature profiles of the PV=Thermal collector system.

FIG. 13. Energy and efficiency profiles of the PV=Thermal collector system.

FIG. 14. Temperature profiles of the integrated PV=Thermal-low temperature desalination system.

043108-14 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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C. Geothermal energy

Geothermal energy sources deliver an energy quantity of 160–200 kW h=m2 annually

which is much higher than photovoltaic (50–100 kW h=m2), biomass (15–45 kW h=m2), and

wind energy resources (11–18 kW h=m2).16 Low grade geothermal source with temperature

of about 60 �C can be used directly to heat the saline water or to maintain a thermal energy

storage tank which can then provide the energy needs to the evaporation chamber

(Fig. 3(c)). The amount of thermal energy supplied by the geothermal source, QG, can be

quantified as

QG ¼ mGcpgðTgi � TgoÞ; (21)

where mG and cpg are the mass flow rate of the geothermal water and Tgi and Tgo are the inlet

and outlet temperatures of the geothermal water, respectively. Saline geothermal energy sources

can be used both as feed (saline water) and heat source.

1. Geothermal Energy Requirements

Geothermal source flow rate for a known freshwater production rate can be estimated using

the following energy balance:

QG ¼ mGcpgðTgi � TgoÞ ¼ mefcpeðTw � TiÞ þ hLðTwÞg; (22)

mG

ðmeÞg¼ R; (23)

where, me is the desired evaporation rate (freshwater production rate, litres=day), cpe is the spe-

cific heat of the water, Tw is the evaporation temperature of the brackish water, Ti is the inlet

temperature of the brackish water, hL is the latent heat of the brackish water at evaporation

temperature, and g is thermal efficiency of the desalination system. R is the ratio of geothermal

source to the mass of freshwater to be evaporated. Based on Eq. (23), for a fixed evaporation

rate of 100 liters=day, the energy requirements and geothermal water flow rates were calculated

(Fig. 16). As expected, the energy and flow requirements increase with the geothermal water

temperatures. This also indicates that when the heat resource is limited, low temperature opera-

tion can provide the benefits of higher thermodynamic efficiency and higher freshwater produc-

tion rates.4

FIG. 15. Evaporation rate and cumulative product of the low temperature desalination system.

043108-15 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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2. Performance of the Low Temperature Desalination System

Temperature profiles for the low temperature desalination system driven by a geothermal

source at 60 �C are shown in Fig. 17. The saline water temperatures in the evaporation chamber

are slightly higher during the sunlight hours due to higher ambient temperatures, and the tem-

perature gradient available between the evaporator and condenser is lower during the daytime

resulting in lower evaporation rates. Temperature gradients between the evaporator and con-

denser are higher during the nonsunlight hours which favor the convection and condensation of

water vapors from the evaporator side to the condenser. From the simulations, it was observed

that the freshwater production rate was around 3.9 liters=h during sunlight hours whereas it was

around 4.8 liters=h during nonsunlight hours. Fig. 17 also shows the temperature profiles for

fluids flowing in and out of the HE1. Saline water enters (cold in) the tube-in-tube heat

exchanger at 25 �C and exits at 36 �C before entering the EC. The brine in (saline water from

EC) temperature is same as the saline water temperature in the EC and is about 51 �C and exits

HE1 at 30 �C. Thus, the heat exchanger operates in the range 75%–85% of thermal efficiency

preheating the saline water entering the EC.

A geothermal water flow rate of 320 kg=h has been considered for numerical simulations

in Fig. 18. Theoretical simulations show that this system can produce up to 100 liters=day of

desalinated water with an average evaporation rate of 4.5 liters=h at a geothermal water temper-

ature of 60 �C. The evaporation rates for geothermal temperatures at 50 �C, 70 �C, and 80 �Care 3 liters=h, 6.5 liters=h, and 8.6 liters=h, respectively. Geothermal sources have potential for

large scale application of the low temperature desalination system. High temperature geother-

mal waters (80–100 �C) are suitable for multi-effect low temperature desalination process to

provide freshwater for small rural communities. They provide continuous source of water and

qualify as a new source of water as the feed itself can be the geothermal water or depending on

the availability of brackish water=seawater sources.

D. Waste heat sources

A general scheme for low temperature desalination system utilizing the process waste heat

is shown in Fig. 3(d). Examples of process waste heat include reject heat from the condenser of

domestic air-conditioning system, exhaust gases from diesel generators, and circulating cooling

water jacket type reactors. The feasibility of the proposed system using a TES system storing

the waste heat rejected by a Li-Br absorption refrigeration system (ARS) has been simulated.8,17

A schematic of this configuration is shown in Fig. 19. In this configuration, the EC area was 5

m2 and the TES was sized to maintain its temperature at 50 �C. An ARS system rated at 3.25

kW (0.975 tons of refrigeration) along with an additional energy input of 208 kJ=kg of desali-

nated water was adequate to produce desalinated water at an average rate of 4.5 kg=h. On a

FIG. 16. Energy and flow rate requirements for the low temperature desalination by geothermal energy

043108-16 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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typical day, the heat demand by the EC (including heat loss) on the TES varied from 8700 to

14 200 kJ=h over a 24-h period; a TES tank volume of 10 m3 was found to be adequate to

maintain its temperature at 50 �C to provide the energy needs of the EC. The operating charac-

teristics of the ARS system were as follows: absorber temperature¼ 28 �C, condenser

temperature¼ 55 �C, evaporator temperature¼ 12 �C, generator temperature¼ 100 �C,

FIG. 17. Low temperature desalination system driven by geothermal energy; (a) EC temperature profiles and (b) HE1 tem-

perature profiles.

FIG. 18. Low temperature desalination system driven by geothermal energy; (a) daily and (b) hourly production rates.

043108-17 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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condenser=generator pressure¼ 15.75 kPa, absorber=evaporator pressure¼ 1.403 kPa, and coef-

ficient of performance¼ 0.72.

In this configuration, the ARS was powered by the solar collectors and augmented by an

auxiliary electric heater. The solar collector system was sized to maintain the generator of the

ARS at 100 �C by maintaining the storage tank of the solar panel at 110 �C. The energy pro-

vided by the auxiliary heater is equal to the difference between the energy required by the

generator and that can be collected from the solar insolation. The energy contributed by solar

collectors from the solar energy is called the solar fraction. For this design, the solar fraction

was 0.4 (40%). The optimum area of the solar collectors for this application was found to be

25 m2. Thermal energy supplied or transferred between the condenser of ARS system and the

TES tank and the heat transfer between TES tank and desalination system depend on the

available temperature gradients and ambient temperature conditions. The TES volume was

determined to be 10 m3 to maintain temperature of the TES water medium within 60.01 �Cduring winter conditions. This unit can be designed to drive the absorption refrigeration unit

round the clock with the solar energy harvested by solar collectors (by increasing the solar

fraction).

Results from the modeling studies discussed above show that the proposed process has the

potential to be driven solely by renewable energy sources or waste heat releases and can be

operated on a continuous basis with moderate yields. More details of the theoretical modeling

results are presented elsewhere.7

E. Specific energy requirements

Specific energy required to produce 1 kg of freshwater for the four energy sources are

shown in Table III. Specific energy requirements include the heat energy used for evaporation

and mechanical energy for pumping the heat source. This table suggests that the above configu-

rations have the potential to produce freshwater either on batch or continuous modes of opera-

tion. The specific energy requirement is also dependent on the mode of operation. The ARS

configuration is the most suitable for domestic applications, since free energy is available from

the ARS condenser and the specific energy requirements are much lower than the remaining

configurations.7

FIG. 19. Low temperature desalination system driven by waste heat rejected by ARS system.

043108-18 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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III. EXPERIMENTAL

A. Direct solar energy

In this section, the theoretical modeling results are compared with experimental results for

the system using direct solar energy. The top of the evaporation chamber was exposed to the

incident solar energy to cause evaporation in the evaporation chamber, thus, the system can be

called solar still under vacuum (SSV) as shown in Fig. 20. Experimental results obtained for

other two configurations (solar still under vacuum with a reflector (SSR) and external photovol-

taic energy (SSPV)) are also presented. All of the results are compared with the traditional solar

still configuration to illustrate the benefits of low temperature desalination unit. The details of

theoretical modeling and experimental results are presented elsewhere.7

1. Temperature and Freshwater Production Profiles

The experimental studies were conducted in summer at engineering research facility in Las

Cruces, USA. The solar insolation varied between 400–1100 W=m2 while the ambient tempera-

tures ranged 15–35 �C during summer. The maximum ambient temperature recorded was 35 �C,

and the maximum temperature of the brackish water in the EC was 52.75 �C. The predicted

FIG. 20. Photo of experimental desalination system driven by direct solar=photovoltaic energy.

TABLE III. Specific energy requirements for the low temperature desalination process using different sources of energy.

Energy source Mode of operation

Thermal energy

required (kJ=kg)

Mechanical energy

required (kJ=kg)

Total Energy

required (kJ=kg)

Solar collectors Batch=continuous 3118 4.1 3122.1

PV=Thermal collectors Batch=continuous 3118 4 3122

Geothermal source Continuous 2934 144 3078

ARS configuration Continuous 194 14 208

043108-19 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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maximum temperature was 52 �C as shown in Fig. 21(a). The correlation between the predicted

and measured EC temperature was satisfactory with r2¼ 0.943, F¼ 2358.2, and p< 0.001. As a

comparison, the maximum saline water temperatures measured for different configurations were

as follows: low temperature desalination process using direct solar energy (solar still configura-

tion, SSV)—50 �C, low temperature desalination process using direct solar energy fitted with an

external reflector (solar still configuration, SSVR)—53 �C, and low temperature desalination

process using solar energy as well as photovoltaic energy (SSPV)—55 �C, respectively, as

shown in Fig. 21(b). These temperatures are lower than those commonly reported for solar stills

which are in the range of 60–75 �C.18,19

The predicted distillate volume during the above test is compared against the measured dis-

tillate volume in Fig. 21(c). Cumulative volume predicted by the model for a 24-h period was

5.25 liters=day m2 while the measured value was 4.95 liters=day m2. The difference (of 5.5%)

in the cumulative distillate volume is mainly due to the assumption that the entire volume of

the vapor distilled on the freshwater side, whereas during the test it was observed that some of

the vapor condensed on the roof of the evaporator and trickled back to the saline water. Corre-

lation between the predicted and measured distillate volume as a function of time was strong

with r2¼ 0.988, F¼ 11,839.4, and p< 0.001.

Daily freshwater production rates for different configurations are shown in Fig. 21(d). The

low temperature desalination process as a SSV produces freshwater of about 5 liters=d m2,

nearly 1.5 to 2 times that of typical solar stills.18,19 This improvement can be attributed to the

reduction in energy losses by the low temperature desalination process. The near-vacuum pres-

sures created by natural means of gravity and barometric head allow for the evaporation of

freshwater to occur at low temperatures resulting in higher energy efficiency. This configura-

tion, when fitted with a reflector (SSVR), produced about 7.5–8 liters=day of distillate which is

three times that of typical solar still. As the solar insolation incident on the solar still was inten-

sified by the reflector, the saline water temperatures rose quickly resulting in evaporation of

freshwater as shown in Fig. 21(d).

Low temperature desalination process powered by photovoltaic energy (SSPV) produced

over 12 l=day when fitted with a reflector. Photovoltaic area required for this configuration was

6 m2. Photovoltaic energy generated during the day is sufficient to produce freshwater of 4–5

liters=day during the night time. The efficiency of the PV modules is 14.2%. The process can

FIG. 21. Saline water temperature and distillate production profiles for different configurations.

043108-20 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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be designed to operate round the clock with addition of an external heat source (during non-

sunlight hours) as in the case of solar collectors. Although, this configuration (SSPV) may not

prove economical, it could be beneficial in arid areas where the need for freshwater and energy

are highly pronounced.

2. Thermal Efficiency and Specific Energy Consumption

Traditional solar stills have a thermal efficiency around 30% and rarely exceed 45%.18,19

Normal solar still (SS) operating with an efficiency of 45%, requires 5040 kJ of thermal energy

per kg of freshwater produced. The proposed process (SSV) operates at higher thermal efficien-

cies with a specific energy consumption of 3900 kJ=kg of freshwater. While the measured ther-

mal efficiency of SSV configuration was 61%, that of the configuration with a reflector (SSVR)

was 75% with a specific energy consumption of 3200 kJ=kg. The specific energy required for

the configuration with photovoltaic energy (SSPV) is only 2800–2900 kJ=kg of freshwater with

thermal efficiencies ranging between 80% and 90%. In the case of traditional solar stills and

SSV, major energy losses occur through the glass cover during sunlight hours. However, for

SSPV, the glass cover can be covered with insulation during non-sunlight hours to reduce the

energy losses through the glass cover. Additionally, lower ambient temperatures during non-

sunlight hours favor the convection and condensation of freshwater vapors from the evaporation

chamber to the condenser side.4 Specific energy consumption for different operational modes

are summarized in Table IV.5

B. Recovery of potable water from secondary effluent using low grade waste heat

1. Using Low Grade Heat Source

Feasibility of running the low temperature desalination process using a low grade thermal

source was demonstrated. A hot water tank was used as heat source in this configuration.20 The

design of the unit was slightly modified in this configuration to integrate the evaporation and

condensation sides of the desalination unit. The condenser plate was arranged at the top of the

evaporation chamber to dissipate the heat of condensation to the ambient, thus reducing the

footprint of the unit. During these tests, the circulation rate of the hot water was maintained at

9 kg=h, while the temperature of the source was varied between 50 �C�70 �C. Typical tempera-

ture profiles recorded over 24 h operation with continuous operation mode are presented in

Fig. 22. Results from these tests showed that the low temperature desalination system operated

with higher efficiency at lower evaporation temperatures since the losses to the ambient are

reduced. At higher evaporation temperatures, the heat dissipation rate depends on the condenser

surface area available and, thus mass of water evaporated. Hourly freshwater production rates

varied between 77–91 ml=h. Thermal energy supplied through the hot water source for tempera-

ture range 50–70 �C varied between 355 and 395 W, while thermal efficiency declined from

53% to 47%. The effect of cooling the condenser surface was tested with a small flow rate of

cooling water of 500 ml=h which was collected at the bottom of the condenser. An increase in

TABLE IV. Specific energy requirements for the low temperature desalination process by different configurations using

solar energy.

Experiment Description

Mode of

operation

Specific energy

requirement (kJ=kg)

Case 1 Direct solar energy Batch 3900

Case 2 Direct solar energy with a reflector Batch 3118

Case 3 Solar energy during sunlight hours, photovoltaic

energy during non-sunlight hours

Continuous 2926

Case 4 Solar and photovoltaic energy together Batch 3325

043108-21 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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thermal efficiency of 10–15% was observed with cooling. Thermal efficiency increased from a

maximum value of 53%–67% for heat source temperature at 50 �C.

The energy efficiency can be improved further with adequate insulation between the evapo-

rator basin and condenser top, addition of fins on the condenser plate for more efficient heat

dissipation and external cooling by recycling the product water. This configuration can be

scaled to utilize low grade heat sources for large scale applications. As an illustration, a case

study for the City of Las Cruces wastewater treatment plant is presented in the next sub-section.

The source water contained biochemical oxygen demand (BOD), total dissolved solids (TDS),

total suspended solids (TSS), nitrates, nitrites, chlorides, and coliform bacteria. However, the

process was able to achieve more than 90% reductions for each of the above contaminants as

shown in Table V. The process produces high quality distillate with TDS< 50 ppm which is

suitable for many non-potable uses.

2. Case Study

The City of Las Cruces treats an average of 10 million gallons per day (MGD) of waste-

water. The wastewater treatment plant has anaerobic sludge digester in place to process the

FIG. 22. Temperature profiles of the low temperature desalination system driven by low grade heat source.

TABLE V. Characteristics of secondary effluent and product water.

Low temperature desalination

using low grade heat

Parameter Secondary effluent Recovered water % Reduction

USEPA drinking

water standard

BOD (mg=l) 12.7 — — —

TDS (mg=l) 783 16 98 500

TSS (mg=l) 8 0.3 96.2 —

Nitrate (as N mg=l) 2.6 <0.1 96.2 10

Nitrites (as N mg=l) 2.6 <0.1 96.2 1

NH3 (as N mg=l) 22.7 1.57 93.1 —

Chlorides (mg=l) 0.5 0 100 4

Coliform (cfu) 110 0 100 0

pH 7.6 7.6 — 6.5–8.5

043108-22 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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biomass. The anaerobic digester produces biogas which can generate up to 350 kW of energy

on a daily basis.21 A multi-effect low temperature unit demonstrated in this study with a gain

to output ratio (GOR)¼ 5 would require a specific energy consumption of 470 kJ=kg of pota-

ble-quality water produced. A total volume of 17 000 gal=day of freshwater can be produced

utilizing the energy generated by the biogas. This freshwater can be used for process cooling

operations, plant maintenance, or cooling and heating applications saving the water and heating

bills for the wastewater treatment plant or can be sold to other industrial or irrigation

applications.

IV. CONCLUSIONS

Modeling and experimental results are presented to show that the proposed low temperature

desalination system has potential to utilize renewable energy sources for both small and large

scale applications. Sustainable use of these energy sources has been studied both by theoretical

and experimental studies. Conclusions from the studies are as follows:

1. Solar collectors can be used with and without thermal energy storage tank. A solar collector

area of 15 m2 is sufficient to produce 100 liters=day of freshwater from the low temperature

desalination system. Inclusion of TES requires additional solar collector area; however, they

are beneficial to compensate the temporary effects of clouding and to ensure continuous supply

of freshwater. The configuration without TES requires a water storage tank because if energy

cannot be stored, water has to be produced when energy is available and stored for future uses.

The configuration without TES is more susceptible to changes in the daily solar insolation and

ambient temperatures.

2. Photovoltaic thermal collectors can be successfully employed to provide both the electri-

cal and thermal energy needs of a household. The electrical efficiency of the PV=T col-

lectors increases with circulating fluid which again provides the heat source to the low

temperature desalination system. 30 m2 PVT collector area can provide a household with

120 liters=day of freshwater and 14–18 kW h of electricity for daily needs. This configu-

ration is suitable for remote and rural applications where scarcity of both energy and

freshwater is high.

3. Geothermal water sources can be utilized to drive the low temperature desalination system

round the clock. Freshwater production rate and required geothermal source flow rate vary with

the available geothermal source temperature. Geothermal sources can be easily applied to large

scale applications. A flow rate of 320 kg=h at 60 �C was found to be sufficient to drive the low

temperature desalination system round the clock to produce 100 liters=day. Provision of TES is

optional and needs to be evaluated in connection with the pumping costs and capital costs of

the TES tank.

4. Utilizing low grade waste heat sources is a green approach to desalination. An absorp-

tion refrigeration system has been illustrated as an example of potential low grade heat

source. Theoretical simulations show that an ARS system rated at 3.25 kW (0.975 tons

of refrigeration) along with an additional energy input of 208 kJ=kg of desalinated water

is adequate to produce desalinated water at an average rate of 4.5 liters=h. The solar col-

lector and TES tank volumes required for this application are 25 m2 and 10 m3,

respectively.

Experimental results obtained by different test configurations complement the theoretical

analysis and prove the operational feasibility of the low temperature desalination system. The

low temperature desalination process was configured in different ways to capture and utilize

the solar energy source to the best during the day. Experimental results prove that the renew-

able energy sources are best utilized when they are coupled with energy-efficient desalination

processes. Additionally, a sustainable use of waste heat release to drive the low temperature

desalination system to recover potable quality water from the secondary effluent of local

waste water treatment facility was evaluated with a case study. Theoretical and experimental

results confirm that the low temperature desalination system is suitable for remote applications

043108-23 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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where there is no electrical grid. However, the benefit of utilizing natural vacuum principle to

save mechanical energy needs has to be studied on a large scale basis to validate the process

feasibility.

NOMENCLATURE

A Area (m2)

c Specific heat (kJ=kg K)

C Concentration (%)

d Diameter of the circulating fluid tube (m)

h Latent heat, heat transfer coefficient (kJ=kg)

f Concentration factor (-)

F Heat removal factor (dimensionless), packing factor (%)

I Solar insolation (kJ=h m2)

m,M Mass (kg)

Nu Nusselt number (-)

P Pressure (-)

Pr Prandtl number (-)

q Evaporation energy (kJ=h)

Q Energy or useful energy (kJ=h)

Re Reynolds number (-)

t Time (h,s)

T Temperature (K)

U Heat loss=transfer coefficient (kJ=h m2 K)

v Volume of the saline water or storage tank (m3)

x Length of the circulating fluid route (m)

Greek

a Absorptivity (-)

am Experimental coefficient (10�7–10�6) (kg K0.5=m2 Pa s)

e Radiation factor (-)

g Efficiency (%)

s Transmitivity of glass (-)

q Density (kg=m3)

r Stefan-Boltzmann constant (5.7� 10�8) (W=m2 K4)

Subscripts

a Ambient

c Collector, cell, specific heat

e Evaporation, electrical

EC Evaporation chamber

f Fluid

gi Geothermal in

go Geothermal out

G,g Geothermal, glass

i,in Inlet, supply

l,ls,lo,lp,L Latent heat, losses

p,PVT Absorber plate, photovoltaic thermal collector

pg Glass, geothermal

power Efficiency of thermal power plant

r,R Recycle, ratio

043108-24 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)

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s,sc Saline water, supply, surface, solar

th Thermal

u Useful energy

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043108-25 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)

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